pager: set an attribute on ui indicating that a pager is active

A subsequent patch will teach the color extension to do different things
depending on whether a pager is active. This patch leaves a breadcrumb
on the ui instance to allow it do that that.

hgext/pager.py

@@ -149,6 +149,8 b' def uisetup(ui):'
                     usepager = True
                     break
 
+        setattr(ui, 'pageractive', usepager)
+
         if usepager:
             ui.setconfig('ui', 'formatted', ui.formatted(), 'pager')
             ui.setconfig('ui', 'interactive', False, 'pager')